Positive electrode for lithium secondary battery, method for manufacturing the same, and lithium secondary battery containing the same

Abstract

The present invention relates to a positive electrode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same. More specifically, because the positive electrode for a lithium secondary battery has a structure including a low-load wet positive electrode active material layer that does not increase the moisture content in the battery and a dry positive electrode active material layer prepared by a dry process, a lithium secondary battery with improved capacity, overvoltage, and lifespan characteristics can be manufactured compared to batteries having a positive electrode including only a wet positive electrode active material layer or only a dry positive electrode active material layer.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 2021-0058926, filed on May 7, 2021, and Korean Patent Application No. 2022-0052596, filed on April 28, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a positive electrode for a lithium secondary battery, a method for manufacturing the positive electrode, and a lithium secondary battery comprising the positive electrode. Background Art

[0003] Recently, with the rapid development in the fields of electronic devices and electric vehicles, the demand for secondary batteries has been increasing. In particular, with the trend of miniaturization and lightweighting of portable electronic devices, the demand for secondary batteries with high energy density that can cope with them is growing.

[0004] Among secondary batteries, lithium-sulfur secondary batteries use sulfur compounds with sulfur-sulfur bonds as the positive electrode active material and the following as the negative electrode active material: alkali metals such as lithium; carbon materials in which metal ions such as lithium ions are intercalated and deintercalated; or silicon or tin alloyed with lithium. Specifically, in lithium-sulfur secondary batteries, during discharge (a reduction reaction), the sulfur-sulfur bonds break, causing the sulfur oxidation number to decrease. During charge (an oxidation reaction), the sulfur oxidation number increases, causing the sulfur-sulfur bonds to reform. This oxidation-reduction reaction allows electrical energy to be stored and generated.

[0005] In particular, in the case of lithium-sulfur secondary batteries, sulfur, used as the cathode active material in lithium-sulfur secondary batteries, has a theoretical energy density of 1,675 mAh / g, which is approximately five times that of the cathode active materials used in conventional lithium secondary batteries. Consequently, these batteries are capable of exhibiting both high power and high energy density. Furthermore, because sulfur is inexpensive, abundant, and readily available, and is environmentally friendly, lithium-sulfur secondary batteries are attracting attention as energy sources not only for portable electronic devices but also for medium- and large-scale devices such as electric vehicles.

[0006] Since the conductivity of sulfur is 5×10 -30 S / cm, a non-conductor with no electrical conductivity, poses a problem of difficulty in transferring electrons generated by electrochemical reactions. Therefore, sulfur is combined with a conductive material such as carbon that can provide electrochemical reaction sites, and the resulting sulfur-carbon composite is used.

[0007] In order to use the sulfur-carbon composite as a positive electrode active material, a method of manufacturing a positive electrode through a slurry process in which the positive electrode active material is prepared into a slurry together with a conductive material and a binder, and then the slurry is applied to a current collector is generally used.

[0008] However, the positive electrode produced by this slurry process has a problem in that, because a conductive material and a binder are used in preparing the slurry, the loading amount of the positive electrode active material in the positive electrode is reduced, resulting in a decrease in energy density. In addition, because the slurry process includes complicated steps such as mixing, coating, drying, and rolling, there is a problem of increased time and cost.

[0009] Therefore, it is necessary to develop a technology that can produce a high-load positive electrode for lithium secondary batteries by a simple method.

[0010] Prior art literature

[0011] [Patent Document]

[0012] (Patent Document 1) Korean Patent Publication No. 2018-0055230

[0013] (Patent Document 2) Japanese Patent Application No. 2018-113142 Summary of the Invention

[0014] [Technical Issues]

[0015] Therefore, the inventors of the present invention conducted various studies to solve the above-mentioned problems, and confirmed that a lithium secondary battery comprising a positive electrode formed by stacking a wet positive electrode active material layer prepared by a wet method and a dry positive electrode active material layer prepared by a dry method has excellent capacity, high rate characteristics, overvoltage and life characteristics, thereby completing the present invention.

[0016] Therefore, an object of the present invention is to provide a positive electrode for a lithium secondary battery and a method for manufacturing the same, the positive electrode having improved battery capacity, high rate characteristics, overvoltage, and lifespan characteristics.

[0017] Another object of the present invention is to provide a lithium secondary battery comprising the positive electrode for a lithium secondary battery.

[0018] [Technical solution]

[0019] In order to achieve the above object, the present invention provides a positive electrode for a lithium secondary battery, wherein the positive electrode for the lithium secondary battery comprises:

[0020] positive electrode current collector;

[0021] a wet positive electrode active material layer formed on one surface of the positive electrode current collector; and

[0022] a dry-type positive electrode active material layer formed on the wet-type positive electrode active material layer,

[0023] wherein the wet positive electrode active material layer comprises a sulfur-carbon composite, a binder and a conductive material, and

[0024] The dry positive electrode active material layer is composed of a sulfur melt containing carbon.

[0025] In addition, the present invention provides a method for manufacturing a positive electrode for a lithium secondary battery, the method comprising the following steps:

[0026] (1) applying a positive electrode slurry containing a sulfur-carbon composite, a binder, and a conductive material onto one surface of the positive electrode current collector to form a wet positive electrode active material layer; and

[0027] (2) A dry-type positive electrode active material layer as a self-standing film-type positive electrode material is attached to one surface of the wet-type positive electrode active material layer.

[0028] In addition, the present invention provides a lithium secondary battery comprising: the above-mentioned positive electrode for lithium secondary battery; a negative electrode comprising lithium metal or a lithium alloy; a separator located between the positive electrode and the negative electrode; and an electrolyte impregnated with the positive electrode, the negative electrode and the separator.

[0029] [Beneficial Effects]

[0030] Compared with a conventional positive electrode for a lithium secondary battery comprising only a wet positive electrode active material layer, the positive electrode for a lithium secondary battery according to the present invention can exhibit improved effects in capacity, overvoltage, high rate characteristics and life characteristics due to the stacked structure of the wet positive electrode active material layer and the dry positive electrode active material layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Longitudinal cross sections of positive electrodes for lithium-sulfur secondary batteries prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively, are shown.

[0032] Figure 2a and 2b are graphs showing changes in specific capacity and voltage during charge and discharge of lithium-sulfur secondary batteries manufactured in Example 1, Comparative Example 1, and Comparative Example 2, respectively.

[0033] Figure 3 Graphs showing lifespan characteristics of lithium-sulfur secondary batteries manufactured in Example 1, Comparative Example 1, and Comparative Example 2, respectively. DETAILED DESCRIPTION

[0034] Hereinafter, the present invention will be described in more detail.

[0035] The terms and words used in this specification and claims should not be construed as limited to ordinary or dictionary terms, and should be interpreted as meanings and concepts consistent with the technical ideas of the present invention on the basis that the inventor can appropriately define the concepts of the terms to describe the principles of his invention in the best possible manner.

[0036] The terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the present invention. Unless the context clearly indicates otherwise, the singular forms "a", "an", and "the" include plural referents. It should be understood that the terms "comprising" or "having" as used in this specification are intended to specify the presence of the described features, numbers, steps, operations, components, parts, or combinations thereof, but do not exclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0037] As used herein, the term "wet-type positive electrode active material layer" refers to a positive electrode active material layer used in a general lithium secondary battery, and it can be prepared by a process including the following steps: coating a positive electrode slurry prepared by mixing and dispersing a positive electrode active material, a binder and a conductive material in a solvent on a positive electrode collector, drying and rolling.

[0038] As used herein, the term "dry-type positive electrode active material layer" refers to a positive electrode active material layer made only of sulfur and a porous carbon material, and can be formed in the form of a self-standing film by heat-treating the sulfur and carbon materials and then pressurizing them. Specifically, the dry-type positive electrode active material layer is composed of a carbon-containing sulfur melt in which a porous carbon material is dispersed within the sulfur melt.

[0039] In the present invention, the term "porosity" refers to the ratio of the volume occupied by pores in a structure to the total volume, and its unit is %. In the present invention, there is no particular limitation on the measurement of porosity. For example, according to one embodiment of the present invention, the volume of micropores and mesopores can be measured by, for example, the Brunauer-Emmett-Teller (BET) measurement method or Hg porosimeter.

[0040] Positive electrode for lithium secondary battery

[0041] The present invention relates to a positive electrode for a lithium secondary battery, which comprises a wet positive electrode active material layer and a dry positive electrode active material layer, wherein the positive electrode for a lithium secondary battery comprises: a positive electrode collector; a wet positive electrode active material layer, wherein the wet positive electrode active material layer is formed on one surface of the positive electrode collector; and a dry positive electrode active material layer, wherein the dry positive electrode active material layer is formed on the wet positive electrode active material layer, wherein the wet positive electrode active material layer comprises a sulfur-carbon composite, a binder and a conductive material, and wherein the dry positive electrode active material layer is composed of a carbon-containing sulfur melt.

[0042] Generally, because dry electrodes manufactured using a dry process do not contain a binder, which acts as a resistive element, they have lower internal resistance and smaller overvoltage than wet electrodes, resulting in superior capacity. Furthermore, because dry electrodes have a lower moisture content than wet electrodes, the likelihood of negative electrode degradation due to moisture is lower, resulting in superior lifespan characteristics. However, conventional dry electrodes have high contact resistance between the dry electrode and the current collector, which poses a problem in terms of pouch-type battery performance.

[0043] Since the positive electrode for a lithium secondary battery according to the present invention has a structure in which a current collector, a wet-type positive electrode active material layer, and a dry-type positive electrode active material layer are stacked in this order, problems of such conventional dry-type electrodes can be improved.

[0044] First, because the wet positive electrode active material layer is coated on the current collector, the resistance between the current collector and the wet positive electrode active material layer can be reduced. Furthermore, since the dry positive electrode active material layer is introduced onto the wet positive electrode active material layer, sulfur melts during pressing, maintaining connectivity between the wet and dry positive electrode active material layers. If the wet positive electrode active material layer does not contain sulfur, maintaining connectivity between the active material layers is difficult.

[0045] The wet positive electrode active material layer not only facilitates the contact between the current collector and the active material layer, but also has a high porosity, which can promote mass transfer.

[0046] In the present invention, the positive electrode current collector supports a positive electrode active material layer described later, and plays a role in transferring electrons between an external lead and the positive electrode active material layer.

[0047] There are no particular limitations on the positive electrode current collector, as long as it has high conductivity and does not cause chemical changes in the lithium secondary battery. Examples of the positive electrode current collector include copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon, copper or stainless steel surface-treated with carbon, nickel, silver, or the like, and aluminum-cadmium alloys.

[0048] The positive electrode current collector may have a fine concavo-convex structure or a three-dimensional porous structure on the surface of the positive electrode current collector to enhance the bonding force with the positive electrode active material layer. Therefore, the positive electrode current collector may include various forms such as films, sheets, foils, nets, meshes, porous bodies, foams, and non-woven fabrics.

[0049] In the present invention, the wet-type positive electrode active material layer may include a sulfur-carbon composite as a positive electrode active material, a binder, and a conductive material.

[0050] Based on the total weight of the wet positive electrode active material layer, the content of the positive electrode active material may be 40% to 80% by weight. Specifically, the content of the positive electrode active material may be 40% by weight or more or 50% by weight or more, and may be 70% by weight or less or 80% by weight or less. If the content of the positive electrode active material is less than 40% by weight, the connectivity between the wet positive electrode active material layer and the dry positive electrode active material layer may be insufficient. If the content of the positive electrode active material exceeds 80% by weight, the mass transfer resistance may increase.

[0051] The sulfur-carbon composite refers to a composite in which sulfur is supported on a porous carbon material. For example, the sulfur-carbon composite may be in a state in which sulfur is attached to or coated on the surface of the porous carbon material. Furthermore, the sulfur-carbon composite may be in a state in which sulfur is also attached to, fills, or coats the internal pores of the porous carbon material; or in a state in which sulfur permeates and adheres to the interior of the porous carbon material.

[0052] In addition, the sulfur may be at least one selected from the following: inorganic sulfur (S8), Li2S n (n≥1, n is an integer), organic sulfur compounds and carbon-sulfur polymers ((C2S x ) n , 2.5≤x≤50, n≥2, x and n are integers).

[0053] Furthermore, the sulfur content, based on the total weight of the sulfur-carbon composite, may be 50% by weight or greater, 55% by weight or greater, or 60% by weight or greater, and may be 70% by weight or less, 75% by weight or less, or 80% by weight or less. If the sulfur content is less than 50% by weight, the proportion of sulfur acting as an electrochemically active material decreases, and the sulfur coating formed on the surface of the porous carbon material may become thinner, making it difficult to properly form the sulfur-carbon composite. Alternatively, the amount of sulfur contained within the porous carbon material may decrease, potentially reducing the capacity of the battery. Furthermore, if the sulfur content exceeds 80% by weight, the non-conductive sulfur may hinder the conductive structure of the porous carbon material, thereby hindering electrochemical activity, potentially limiting the operation of the battery.

[0054] Furthermore, since the porous carbon material has pores or hollow portions formed therein, it can have a property of high specific surface area, and the porous carbon material may be any porous carbon material commonly used in the art.

[0055] The porous carbon material may be, but is not limited to, at least one selected from the group consisting of: graphite; graphene; carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and activated carbon. Preferably, the porous carbon material may be a carbon nanotube.

[0056] In addition, the shape of the carbon nanotubes is not particularly limited, and for example, the carbon nanotubes may have a diameter of 200 nm. 2 / g to 500m 2 / g specific surface area and a particle size of 10 to 30 μm. In this case, the particle size may refer to the length of a portion corresponding to the longest axis of the particle.

[0057] In addition, based on the total weight of the sulfur-carbon composite, the content of the porous carbon material may be 20% by weight or more, 25% by weight or more, 30% by weight or more, or 35% by weight or more, and may be 40% by weight or less, 45% by weight or less, or 50% by weight or less. If the porous carbon material is less than 20% by weight, the surface area and space for sulfur filling, attachment, or coating cannot be fully provided, so that the electrochemical availability (reactivity) of sulfur may be reduced. If the porous carbon material exceeds 50% by weight, the sulfur content is relatively reduced, so that when applied to a lithium secondary battery, the energy density of the battery may be excessively reduced.

[0058] The binder is a component that facilitates bonding between the positive electrode active material and the conductive material, and facilitates bonding to the current collector. The binder may include at least one selected from the group consisting of styrene-butadiene rubber, acrylated styrene-butadiene rubber, acrylonitrile copolymer, acrylonitrile-butadiene rubber, nitrile rubber, acrylonitrile-styrene-butadiene copolymer, acrylic rubber, butyl rubber, fluororubber, polytetrafluoroethylene, polyethylene, polypropylene, ethylene / propylene copolymer, polybutadiene, polyethylene oxide, chlorosulfonated polyethylene, polyvinyl pyrrolidone, polyvinyl pyridine, polyvinyl alcohol, polyvinyl acetate, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, latex, acrylic resin, phenolic resin, epoxy resin, carboxymethyl cellulose, hydroxypropyl cellulose, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl cellulose, cyanoethyl sucrose, polyester, polyamide, polyether, polyimide, polycarboxylate, polycarboxylic acid, polyacrylic acid, polyacrylate, lithium polyacrylate, polymethacrylic acid, polymethacrylate, polyacrylamide, polyurethane, polyvinylidene fluoride, and poly(vinylidene fluoride)-hexafluoropropylene. Preferably, the binder may include at least one selected from the group consisting of styrene-butadiene rubber, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, lithium polyacrylate, and polyvinylidene fluoride.

[0059] In addition, based on the total weight of the wet positive electrode active material layer, the content of the binder may be 1% by weight to 30% by weight. Specifically, the content of the binder may be 1% by weight or more or 3% by weight or more, and may be 15% by weight or less or 30% by weight or less. If the content of the binder is less than 1% by weight, the bonding force between the positive electrode active material and the positive electrode current collector may be insufficient. If the content of the binder exceeds 30% by weight, the bonding force is improved, but the content of the positive electrode active material is reduced by a corresponding amount, thereby reducing the capacity of the battery.

[0060] The conductive material contained in the positive electrode is not particularly limited, as long as it has excellent conductivity without causing side reactions in the internal environment of the lithium secondary battery and without causing chemical changes in the battery. The conductive material can generally be graphite or conductive carbon. For example, as the conductive material, the following substances can be used alone or in combination of two or more: graphite such as natural graphite, artificial graphite; carbon black materials such as carbon black, acetylene black, Ketjen black, Danka black, thermal black, channel black, furnace black, lamp black, thermal black; carbon materials having a crystal structure of graphene or graphite; conductive fibers such as carbon fibers and metal fibers; fluorocarbons; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives, but are not necessarily limited thereto.

[0061] Based on the total weight of the wet positive electrode active material layer, the content of the conductive material may generally be 0.5 wt % to 30 wt %, specifically, the content of the conductive material may be 0.5 wt % or more or 1 wt % or more, and may be 20 wt % or less or 30 wt % or less. If the content of the conductive material is too small, i.e., less than 0.5 wt %, it is difficult to expect the effect of improving the conductivity, or the electrochemical performance of the battery may deteriorate. If the content of the conductive material exceeds 30 wt %, which is excessive, the amount of positive electrode active material may be relatively small, so that the capacity and energy density may be reduced. There is no particular limitation on the method for introducing the conductive material into the positive electrode, and conventional methods known in the art, such as coating on the positive electrode active material, may be used. In addition, as needed, the addition of the above-mentioned conductive material may also be replaced by adding a second conductive coating to the positive electrode active material.

[0062] In the present invention, the term "porosity" refers to the ratio of the volume occupied by pores in a structure to the total volume, and its unit is %. In the present invention, there is no particular limitation on the measurement of porosity. For example, according to one embodiment of the present invention, the volume of micropores and mesopores can be measured by a Bruer-Emmett-Teller (BET) measurement method or an Hg porosimeter.

[0063] In addition, the density of the wet positive electrode active material layer can be 0.2 g / cm 3 Up to 1.4g / cm 3 Specifically, the density of the wet positive electrode active material layer can be 0.2 g / cm 3 Above, 0.3g / cm 3 Above or 0.5g / cm 3 Above, and can be 1.0g / cm 3 Below, 1.2g / cm 3 Below or 1.4g / cm 3 If the density is less than 0.2g / cm 3 , the contact resistance may increase, and if the density is greater than 1.4g / cm 3 , the mass transfer resistance may increase.

[0064] In addition, the loading capacity of the wet positive electrode active material layer can be 0.1 mAh / cm 2 Up to 0.5 mAh / cm 2 Specifically, the loading capacity of the wet positive electrode active material layer can be 0.1 mAh / cm 2 Above, 0.2mAh / cm 2 Above or 0.3mAh / cm 2 Above, and can be 0.4mAh / cm 2 Below or 0.5mAh / cm 2If the loading amount of the wet positive electrode active material layer satisfies the above range, it can be beneficial to maintain battery performance. Based on the total loading amount of the dry positive electrode active material layer as described below, the loading amount of the wet positive electrode active material layer can be 2% to 20%. Specifically, the loading amount of the wet positive electrode active material layer can be 2% or more or 3% or more, and can be 10% or less, 15% or less, or 20% or less.

[0065] In addition, the porosity of the wet positive electrode active material layer may be 30% to 90%, specifically, 30% or more, 50% or more, 60% or more, or 70% or more, and 80% or less, 85% or less, or 90% or less. In addition, if the porosity satisfies such a porosity range, the performance of the battery can be well maintained while maintaining the durability of the battery.

[0066] In addition, the weight of the wet positive electrode active material layer may be 1% by weight to 15% by weight, specifically, 1% by weight or more, 3% by weight or more, or 5% by weight or more, and may be 10% by weight or less, 13% by weight or less, or 15% by weight or less, based on the total weight of the wet positive electrode active material layer and the dry positive electrode active material layer. If the weight of the wet positive electrode active material layer is less than 1% by weight, the adhesive force between the current collector and the dry positive electrode active material layer may be reduced, and if the weight of the wet positive electrode active material layer exceeds 15% by weight, it may act as a resistor of the battery due to the binder contained in the wet positive electrode active material layer.

[0067] In addition, based on the total thickness of the wet positive electrode active material layer and the dry positive electrode active material layer, the thickness of the wet positive electrode active material layer may be 1% to 30%, specifically, it may be 1% or more, 5% or more, or 10% or more, and may be 20% or less, 25% or less, or 30% or less. If the thickness of the wet positive electrode active material layer is less than 1%, the adhesion between the current collector and the dry positive electrode active material layer may be reduced. If the thickness of the wet positive electrode active material layer exceeds 30%, it may act as a resistor of the battery due to the binder contained in the wet positive electrode active material layer.

[0068] In the present invention, the dry-type positive electrode active material layer may include a sulfur melt containing carbon, wherein the porous carbon material is dispersed in the sulfur melt.

[0069] The dry-type positive electrode active material layer may contain the positive electrode material in the form of a self-standing film composed only of the sulfur melt containing carbon without containing the binder and the conductive material contained in the wet-type positive electrode active material layer.

[0070] Because the positive electrode material in the form of a self-supporting film is prepared by a dry method using sulfur and porous carbon material as raw materials, the positive electrode material only contains sulfur and porous carbon material, thereby having the advantage of a high load capacity when used as a positive electrode. In addition, the dry method can save a series of processes such as mixing, defoaming, coating, drying and rolling required for conventional slurry processes, thereby reducing process costs. In addition, because the dry method does not use slurry, the carbonaceous sulfur melt manufactured by the dry method does not contain any adhesive, thus fundamentally eliminating the deterioration of battery performance caused by adhesive resistance. In addition, because the carbonaceous sulfur melt manufactured by the dry method does not contain conductive material at all, the problem of formability degradation caused by the lack of cohesion of conductive material can be minimized.

[0071] Furthermore, the positive electrode material in the form of a self-standing film is connected by sulfur melt formed on the surface of the porous carbon material, while the porous carbon material forms the positive electrode material skeleton, thereby forming a self-standing film. Carbon nanotubes, due to their structural characteristics, have more connection points in the porous carbon material, which can be more advantageous in forming a self-standing film. Specifically, because the carbon nanotubes have a shape with an aspect ratio greater than 1, they can be advantageously connected to each other to form a self-standing film.

[0072] In addition, the positive electrode material in the form of a self-supporting film can be a positive electrode material having an adhesive force of 10 gf / cm or more in the positive electrode material after the electrode is pressed and formed. The adhesive force is caused by the property of sulfur melting and aggregating with the surrounding sulfur during the pressing process, and if the adhesive force of the positive electrode material is less than 10 gf / cm, it may be difficult to form an electrode due to insufficient adhesive force between the positive electrodes. Specifically, the adhesive force can be more than 10 gf / cm, more than 15 gf / cm, more than 20 gf / cm, more than 25 gf / cm, more than 30 gf / cm, or more than 35 gf / cm. In addition, the upper limit of the adhesive force can be less than 50 gf / cm, less than 60 gf / cm, less than 70 gf / cm, less than 80 gf / cm, less than 90 gf / cm, or less than 100 gf / cm, but is not limited thereto. The higher the adhesive force in the positive electrode material, the better the formability, durability, and battery performance.

[0073] In addition, the carbon-containing sulfur melt contained in the dry positive active material layer can be composed of 50% to 80% by weight of sulfur and 20% to 50% by weight of porous carbon material. Based on the total weight of the carbon-containing sulfur melt, the sulfur content can be 50% by weight or more, 55% by weight or more, or 60% by weight or more, and the content can be 70% by weight or less, 75% by weight or less, or 80% by weight or less. If the sulfur content is less than 50% by weight, the proportion of sulfur as an electrochemically active material is reduced, and the thickness of the sulfur melt formed on the surface of the porous carbon material becomes thinner, thereby making it difficult for the carbon-containing sulfur melt to be properly formed, or the amount of sulfur may be reduced and reduce the capacity of the battery. In addition, if the sulfur content exceeds 80% by weight, the non-conductive sulfur will hinder the conductive structure of the porous carbon material, thereby hindering the electrochemical activity, which may limit the operation of the battery. When the sulfur content in the sulfur melt containing carbon is 50 to 80 wt %, a self-standing positive electrode can be well formed because the positive electrode material can exhibit strong self-cohesion and the porous carbon material can be well dispersed in the sulfur melt.

[0074] Furthermore, the porosity of the dry-type positive electrode active material layer may be 68% or less, 65% or less, 60% or less, or 55% or less, and may be 45% or more, or 50% or more. If the porosity of the dry-type positive electrode active material layer exceeds 68%, the durability of the positive electrode may be reduced. If the porosity of the dry-type positive electrode active material layer is less than 45%, the space for electrochemical reactions to occur in the pores becomes narrower, making it difficult for the battery to function properly.

[0075] In addition, the loading capacity of the dry positive active material layer can be 2.5 mAh / cm 2 Up to 5.0 mAh / cm 2 If the loading amount of the dry-type positive electrode active material layer satisfies the above range, it can be well used to maintain the performance of the battery.

[0076] In addition, based on the total weight of the wet positive electrode active material layer and the dry positive electrode active material layer, the weight of the dry positive electrode active material layer may be 85 wt % to 99 wt %, specifically, 85 wt % or more, 87 wt % or more, or 90 wt % or more, and may be 95 wt % or less, 97 wt % or less, or 99 wt % or less. If the weight of the dry positive electrode active material layer is less than 85 wt %, since the weight of the wet positive electrode active material layer is relatively large, it may act as a resistor of the battery due to the binder contained in the wet positive electrode active material layer. If the weight of the dry positive electrode active material layer exceeds 99 wt %, the adhesive force between the current collector and the dry positive electrode active material layer may be reduced.

[0077] In addition, the thickness of the dry positive electrode active material layer may be 70% to 99% based on the total thickness of the wet positive electrode active material layer and the dry positive electrode active material layer, specifically, it may be 70% or more, 75% or more, or 80% or more, and may be 90% or less, 95% or less, or 99% or less. If the thickness of the dry positive electrode active material layer is less than 70%, since the weight of the wet positive electrode active material layer is relatively large, it may act as a resistor of the battery due to the binder contained in the wet positive electrode active material layer. If the thickness of the dry positive electrode active material layer exceeds 99% by weight, the adhesive force between the current collector and the dry positive electrode active material layer may be reduced.

[0078] Method for manufacturing positive electrode for lithium secondary battery

[0079] The present invention also relates to a method for manufacturing a positive electrode for a lithium secondary battery, the method comprising the following steps: (1) applying a positive electrode slurry containing a sulfur-carbon composite, a binder and a conductive material to one surface of the positive electrode collector to form a wet positive electrode active material layer; and (2) attaching a dry positive electrode active material layer as a self-standing film-type positive electrode material to one surface of the wet positive electrode active material layer.

[0080] Hereinafter, the present invention will be described in more detail with respect to each step.

[0081] In the present invention, in step (1), a positive electrode slurry containing the sulfur-carbon composite, the binder, and the conductive material may be applied to one surface of the positive electrode current collector to form a wet positive electrode active material layer. In this case, the properties of the sulfur-carbon composite, the binder, the conductive material, and the positive electrode current collector are as described above.

[0082] Specifically, the positive electrode slurry can be coated on the positive electrode collector and vacuum dried to form a positive electrode for a lithium secondary battery. The positive electrode slurry can be coated on the positive electrode collector with an appropriate thickness according to the viscosity of the slurry and the thickness of the positive electrode to be formed, preferably in the range of 10 nm to 1 μm.

[0083] In this case, there is no limitation on the method of coating the positive electrode slurry, and the method of coating the slurry may include, for example, blade coating, dip coating, gravure coating, slot die coating, spin coating, comma coating, rod coating, reverse roll coating, screen coating, cap coating, and the like.

[0084] In addition, the slurry for the positive electrode may be prepared by mixing the sulfur-carbon composite, the binder, and the conductive material with a solvent and stirring.

[0085] As solvent, use a solvent that can uniformly disperse sulfur-carbon composite, adhesive and conductive material. As such solvent, water is most preferably water as an aqueous solvent, in which case water can be double distilled water (DW) or triple distilled water (deionized water (DIW)). However, the present invention is not necessarily limited thereto, and if necessary, lower alcohols that can be easily mixed with water can be used. The lower alcohols can be methanol, ethanol, propyl alcohol, isopropyl alcohol and butanols etc., and preferably they can be used in the form of a mixture with water.

[0086] Furthermore, mixing for preparing the slurry for the positive electrode can be performed by stirring in a conventional manner using a conventional mixer such as a paste mixer, a high-speed shear mixer, or a homomixer.

[0087] The positive electrode may be manufactured by coating the prepared positive electrode slurry on a positive electrode collector and then drying it, and if necessary, in order to increase the electrode density, it may be manufactured by compression molding on the positive electrode collector.

[0088] In the present invention, in step (2), a dry-type positive electrode active material layer as a self-standing film type positive electrode material may be attached to one surface of the wet-type positive electrode active material layer.

[0089] In the present invention, a self-standing film-type positive electrode material is prepared by a process comprising the steps of: (a) mixing sulfur with a porous carbon material; (b) heat-treating the mixture formed in step (a); and (c) filling the sulfur-carbon composite formed in step (b) into a container and then pressing it.

[0090] In step (a), a mixture of sulfur and a porous carbon material as raw materials may be formed. In this case, the types and suitable weight ranges of sulfur and porous carbon material are the same as those described above.

[0091] Furthermore, in step (b), the mixture formed in step (a) may be heat-treated to form a sulfur-carbon composite.

[0092] When the mixture of sulfur and the porous carbon material is heated, the sulfur becomes liquid and enters the porous carbon material or is coated or adhered to the surface, causing the sulfur to be loaded or filled in the porous carbon material and / or forming a coated sulfur-carbon composite. For example, if the porous carbon material is a carbon nanotube, the liquid sulfur can be absorbed into the carbon nanotube through capillary action, thereby allowing the sulfur to be loaded on the carbon nanotube.

[0093] The heat treatment can be performed at a temperature above the melting point of sulfur. For example, the heat treatment temperature can be 130°C or higher, 140°C or higher, or 150°C or higher, and can be 160°C or lower, 165°C or lower, or 170°C or lower. If the heat treatment temperature is lower than 130°C, the sulfur does not melt, making it difficult to form a composite in which the sulfur is supported or coated on the carbon material. If the heat treatment temperature is higher than 170°C, although the sulfur-carbon composite can be produced, sulfur volatilization may occur, resulting in sulfur loss and degradation of the production equipment.

[0094] The heat treatment time may be any time sufficient to melt sulfur and support it on the porous carbon material, and may be 25 minutes or more, 30 minutes or more, and 40 minutes or less, 45 minutes or less, or 50 minutes or less.

[0095] Furthermore, in step (c), by charging the sulfur-carbon composite formed in step (b) into a container and then pressurizing it, a positive electrode material in the form of a self-standing film made of a sulfur melt containing carbon can be manufactured.

[0096] Sulfur-carbon composites exhibit strong self-adhesion under pressure. Specifically, under pressure, the sulfur on the surface of the sulfur-carbon composite partially melts, providing connectivity between the composites and exhibiting strong self-adhesion. Therefore, when pressure is applied to the particulate sulfur-carbon composite, the sulfur melts to form a sulfur melt, and a carbon-containing sulfur melt is formed, in which the carbon material is dispersed. A self-supporting film is formed because cohesion is generated between the dispersed carbon material particles, the carbon material acts as a framework, and its inherent flexibility.

[0097] The pressure during pressurization can be sufficient to form a self-supporting film by generating sufficient cohesive force between the sulfur-carbon composite. For example, the pressure during pressurization can be 0.8 MPa or more, 0.9 MPa or more, or 1 MPa or more, and can be 5 MPa or less, 8 MPa or less, 10 MPa or less, 13 MPa or less, or 15 MPa or less. If the pressure during pressurization is less than 0.8 MPa, the cohesive force between the sulfur-carbon composite is weak, and a self-supporting film made of the carbon-containing sulfur melt may not be formed. If the pressure during pressurization exceeds 15 MPa, the porosity of the positive electrode material is too low, and the structure of the electrode may collapse.

[0098] lithium secondary batteries

[0099] Furthermore, the present invention provides a lithium secondary battery including a positive electrode for a lithium secondary battery.

[0100] The lithium secondary battery according to the present invention comprises: a positive electrode; a negative electrode; and an electrolyte interposed therebetween, wherein the positive electrode comprises the positive electrode for a lithium secondary battery according to the present invention.

[0101] The positive electrode is as described above and includes a self-standing film type positive electrode material in a positive electrode active material layer, characterized in that the positive electrode active material layer and the positive electrode current collector are bonded via a binder.

[0102] In particular, the positive electrode of the present invention can support a larger amount of sulfur than conventional electrodes by including a self-supporting film-type positive electrode material prepared by a dry process that does not require a binder or conductive material in the positive electrode active material layer. Therefore, in the present invention, the sulfur loading in the positive electrode, that is, the mass of sulfur per unit area of ​​the positive electrode active material layer in the positive electrode, can be 3.0 mAh / cm 2 ~5.0mAh / cm 2 Thus, the lithium secondary battery including the positive electrode according to the present invention can exhibit excellent discharge capacity and lifespan characteristics due to the high loading amount of sulfur.

[0103] The negative electrode may be manufactured by forming a negative electrode active material layer containing a negative electrode active material on at least one surface of a negative electrode current collector, or may be a separate negative electrode active material layer (eg, a lithium metal plate, a lithium metal thin film, or a lithium foil).

[0104] The negative electrode current collector serves to support the negative electrode active material layer, and is the same as described with respect to the positive electrode current collector.

[0105] The negative electrode active material layer contains a negative electrode active material, and may further contain a conductive material, a binder, and the like.

[0106] The negative electrode active material may include: a material capable of reversibly inserting or extracting lithium ions (Li + ) material; a material capable of reacting with lithium ions to reversibly form a lithium-containing compound; lithium metal; or a lithium alloy.

[0107] The reversible insertion and removal of lithium ions (Li + ) can be, for example, crystalline carbon, amorphous carbon or a mixture thereof. + ) to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon. Furthermore, the lithium alloy may be, for example, an alloy of lithium and a metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0108] Preferably, the negative electrode active material may be lithium metal, specifically, in the form of a lithium metal film or lithium metal powder.

[0109] There is no particular limitation on the method for forming the negative electrode active material layer, and methods commonly used in the art for forming a layer or film can be used. For example, methods such as compression, coating, or deposition can be used. In addition, the negative electrode of the present invention also includes the case where a thin film of metallic lithium is formed on the metal plate by initial charging after assembling the battery without a lithium thin film on the current collector.

[0110] The conductive material is a material that electrically connects the negative electrode active material and the electrolyte to serve as a movement path of electrons from the current collector to the negative electrode active material, and can be used without limitation as long as it has conductivity.

[0111] For example, as the conductive material, the following substances can be used alone or in combination: graphite such as natural graphite and artificial graphite; carbon black such as Super-P, Danka black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; carbon derivatives such as carbon nanotubes and fullerenes; conductive fibers such as carbon fibers and metal fibers; fluorocarbons; metal powders such as aluminum powder and nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene and polypyrrole.

[0112] The binder is the same as described with respect to the binder for the positive electrode.

[0113] The electrolyte contains lithium ions and serves to cause electrochemical oxidation or reduction reactions at the positive electrode and the negative electrode.

[0114] As the electrolyte, all electrolytes commonly used in lithium secondary batteries can be used.

[0115] For example, lithium salts that can be contained in the electrolyte as an electrolyte can be used without particular limitation, as long as they are commonly used in electrolytes for lithium secondary batteries. For example, the anion of the lithium salt can be any one selected from the following: F - 、Cl - Br - , I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - PF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N -CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2(lithium bis(perfluoroethylsulfonyl)imide, BETI), LiN(CF3SO2)2(lithium bis(trifluoromethanesulfonyl)imide, LiTFSI), LiN(C a F 2a+1 SO2)(C b F 2b+1 SO2) (wherein a and b are natural numbers, preferably 1≤a≤20 and 1≤b≤20), lithium poly[4,4'-(hexafluoroisopropylidene)diphenoxy]sulfonimide (LiPHFIPSI), LiCl, LiI, LiB(C2O4)2 and LiNO3, among which lithium imide compounds containing a sulfonyl group such as LiTFSI, BETI or LiPHFIPSI may be more preferred.

[0116] In the electrolyte used in the present invention, the organic solvent contained in the electrolyte can be used without restriction, as long as they are commonly used in the electrolyte of lithium secondary batteries. Generally, it is possible to representatively use at least one selected from the following or a mixture of two or more thereof etc.:propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, tetraethylene glycol dimethyl ether (TEGDME), dioxolane (DOL), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite and tetrahydrofuran. Especially, ethylene carbonate and propylene carbonate as cyclic carbonate in carbonate organic solvents are high viscosity organic solvents and can be preferably used because their dielectric constant is high and lithium salts are well dissociated in electrolyte. If these cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constant such as dimethyl carbonate and diethyl carbonate in an appropriate ratio, an electrolyte having high conductivity can be prepared, and thus this can be more preferably used.

[0117] In addition to the above-mentioned lithium salt and organic solvent, the electrolyte may further contain a nitric acid-based or nitrous acid-based compound as an additive.

[0118] The present invention has no particular limitation on the nitric acid or nitrous acid compound, but it can be at least one selected from the following: inorganic nitric acid or nitrous acid compound such as lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2), ammonium nitrite (NH4NO2); organic nitric acid or nitrous acid compound such as methyl nitrate, dialkyl imidazole nitrate Guanidine nitrate, imidazole nitrate Pyridine nitrate Ethyl nitrite, propyl nitrite, butyl nitrite, amyl nitrite, octyl nitrite; organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene and combinations thereof, preferably lithium nitrate.

[0119] A separator may be included between the positive electrode and the negative electrode.

[0120] The separator separates or insulates the positive electrode and the negative electrode from each other and enables lithium ions to be transferred between the positive electrode and the negative electrode, and can be made of a porous non-conductive or insulating material. The separator can be used without particular limitation as long as it is used as a separator in a conventional lithium secondary battery. The separator can be an independent component such as a film, or can be a coating added to the positive electrode and / or the negative electrode.

[0121] As the separator, one having low resistance to ion migration of the electrolyte and excellent impregnation ability for the electrolyte is preferred.

[0122] The separator may be made of a porous substrate. Any porous substrate may be used as long as it is a porous substrate commonly used in secondary batteries. A porous polymer film may be used alone or in the form of a laminate. For example, a non-woven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber or a polyolefin-based porous film may be used, but is not limited thereto.

[0123] The present invention has no particular limitation on the material of the porous substrate, and any material can be used as long as it is a porous substrate commonly used in electrochemical devices. For example, the porous substrate may comprise at least one material selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzobis[theta]yl)benzene ...(p-phenylene benzobis[theta azoles) and polyarylates.

[0124] The thickness of the porous substrate is not particularly limited, but may be 1 μm to 100 μm, preferably 5 μm to 50 μm. Although the thickness range of the porous substrate is not particularly limited to the above range, if the thickness is much thinner than the above lower limit, the mechanical properties deteriorate, and thus the separator may be easily damaged during battery use.

[0125] Although the average size and porosity of pores present in the porous substrate are not particularly limited, they may be 0.001 μm to 50 μm and 10% to 95%, respectively.

[0126] The type of the lithium secondary battery as described above is not particularly limited, and may be, for example, a roll type, a stack type, a stack-folding type (including a stack-Z-folding type), or a layer-stack type, preferably a stack-folding type.

[0127] The negative electrode, separator, and positive electrode as described above are sequentially stacked, and an electrolyte is injected to prepare an electrode assembly, which is then placed in a battery case and sealed with a cover plate and a gasket to manufacture a lithium secondary battery.

[0128] In this case, lithium secondary batteries can be divided into a variety of batteries such as lithium-sulfur secondary batteries, lithium-air batteries, lithium-oxide batteries and lithium all-solid-state batteries according to the materials of the positive electrode / negative electrode used, and can be divided into cylindrical, rectangular, coin-shaped, and bag-shaped according to the shape, and can be divided into block type and thin film type according to the size. The structures and preparation methods of these batteries are well known in the art, so their detailed description is omitted.

[0129] In the present invention, because the lithium secondary battery uses a free-standing film-forming cathode material comprising a sulfur-carbon composite as the positive electrode, it can be a lithium-sulfur secondary battery. Lithium-sulfur secondary batteries can use lithium metal as the negative electrode active material. During discharge, lithium-sulfur secondary batteries undergo lithium oxidation at the negative electrode and sulfur reduction at the positive electrode. The reduced sulfur combines with lithium ions removed from the negative electrode, converting them into lithium polysulfide, ultimately leading to the formation of lithium sulfide.

[0130] Furthermore, the present invention provides a battery module including the above-mentioned lithium secondary battery as a unit cell.

[0131] The battery module can be used as a power source for medium or large-sized devices requiring high-temperature stability, long cycle characteristics, high capacity characteristics, and the like.

[0132] Examples of such medium and large devices may include, but are not limited to: power tools powered by battery-driven motors; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc.; electric two-wheeled vehicles, including electric bicycles (E-bikes) and electric scooters (E-scooters); electric golf carts; and power storage systems.

[0133] Preferred Implementation

[0134] Hereinafter, in order to facilitate understanding of the present invention, preferred embodiments of the present invention will be described. However, it will be apparent to those skilled in the art that the following examples are provided to illustrate the present invention and that various variations and modifications can be made within the scope and spirit of the present invention. Such variations and modifications are also within the scope of the appended claims.

[0135] Preparation Example 1: Preparation of sulfur-carbon composite

[0136] Sulfur (S) and carbon nanotubes (CNTs) were uniformly mixed in a weight ratio of 65:35 in a solid state and ball-milled at 100 rpm for 1 hour to prepare a mixture.

[0137] Sulfur-carbon composite (S-CNT) was prepared by heat treating the mixture at 155°C for 35 minutes to load sulfur into the pores of CNT and coat it on the surface. In this case, a carbon composite with a specific surface area of ​​350 m 2 / g of CNT.

[0138] Example 1

[0139] (1) Manufacturing of positive electrode

[0140] The sulfur-carbon composite (S-CNT) obtained in Preparation Example 1, the conductive material, and the binder were mixed in a weight ratio of 90:5:5 to prepare a slurry, which was then coated on an aluminum current collector having a thickness of 20 μm, dried, and rolled to form a wet positive electrode active material layer. In this case, a 300 m 2 CNTs with a specific surface area and a particle size of 20 μm were used as the conductive material, and styrene-butadiene rubber (SBR) was used as the binder.

[0141] The sulfur-carbon composite (S-CNT) of Preparation Example 1 was filled into a mold and then pressurized at a pressure of 10 MPa using a hydraulic press to prepare a self-standing film-type positive electrode material made of a sulfur melt containing carbon.

[0142] A self-standing film type positive electrode material is layered on one surface of the wet positive electrode active material layer and then pressed to form a dry positive electrode active material layer.

[0143] A positive electrode for a lithium secondary battery was manufactured in which an aluminum current collector, a wet-type positive electrode active material layer, and a dry-type positive electrode active material layer were stacked in this order.

[0144] (2) Manufacturing of lithium-sulfur secondary batteries

[0145] The prepared positive electrode and a lithium metal having a thickness of 150 μm as a negative electrode were prepared.

[0146] The electrolyte was prepared by dissolving 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.1 M lithium nitrate (LiNO3) in an organic solvent obtained by mixing tetraethylene glycol dimethyl ether (TEGDME) / dioxolane (DOL) / dimethoxyethane (DME) in a volume ratio of 1:1:1.

[0147] A porous polyethylene separator with a thickness of 20 μm and a porosity of 45% was interposed between the positive electrode and the negative electrode to manufacture an electrode assembly, and the electrode assembly was placed in a case, and then an electrolyte was injected into the case to manufacture a lithium-sulfur secondary battery in the form of a pouch cell.

[0148] Comparative Example 1

[0149] A positive electrode and a lithium-sulfur secondary battery were manufactured in the same manner as in Example 1, except that the dry-type positive electrode active material layer was not formed and only the wet-type positive electrode active material layer was formed.

[0150] Comparative Example 2

[0151] A positive electrode and a lithium-sulfur secondary battery were manufactured in the same manner as in Example 1, except that a dry positive electrode active material layer was formed on one surface of an aluminum current collector instead of a wet positive electrode active material layer. In this case, SBR (styrene-butadiene rubber) as a binder was applied to the aluminum current collector, and then the dry positive electrode active material layer was attached.

[0152] Comparative Example 3

[0153] A dry active material layer was attached to an aluminum current collector in the same manner as in Comparative Example 2, except that no binder was used.

[0154] Experimental Example 1: Evaluation of Physical Properties of Batteries

[0155] For the lithium secondary batteries in the form of pouch-type cells manufactured in Examples and Comparative Examples, the capacity, nominal voltage, moisture content, and lifespan characteristics were measured as follows, and the results are shown in Table 1 below.

[0156] In the case of Comparative Example 3, physical property evaluation was not performed because the dry cathode active material layer itself had no adhesive force and did not adhere to the current collector without using a separate binder, and thus a cathode could not be manufactured.

[0157] (1) Capacity

[0158] After 2.5 cycles of 0.1C charge / 0.1C discharge at 25°C and then 3 cycles of 0.2C charge / 0.2C discharge, the capacity was measured under the conditions of 0.2C charge / 0.3C discharge at temperatures of 25°C and 45°C.

[0159] (2) Nominal voltage

[0160] The nominal voltage is the nominal voltage value used to refer to a given voltage system in the power system. It is generally a value slightly lower than the electromotive force and usually refers to the center value of the discharge curve (the average value of the discharge voltage).

[0161] In this experimental example, the nominal voltage is as follows Figure 2a and 2b The center value of the discharge curve shown in is obtained.

[0162] (3) Moisture content

[0163] The moisture content was measured using a moisture meter (Metrohm 901KF Tirando).

[0164] (4) Lifespan characteristics

[0165] The point at which the capacity retention rate reached 80% of the initial capacity was set as the last cycle of the lifespan.

[0166] Table 1:

[0167]

[0168] Figure 1 Longitudinal cross sections of the positive electrodes of the lithium-sulfur secondary batteries manufactured in Example 1, Comparative Example 1, and Comparative Example 2 are respectively shown.

[0169] like Figure 1As shown, the positive electrode manufactured in Example 1 has a structure in which a current collector (10), a wet positive electrode active material layer (20) and a dry positive electrode active material layer (30) are stacked. The wet positive electrode active material layer (20) exhibits a relatively higher porosity than the dry positive electrode active material layer (30), and the dry positive electrode active material layer (30) exhibits a relatively higher density characteristic than the wet positive electrode active material layer (20). In addition, due to the wet positive electrode active material layer (20), when the current collector (10) and the dry positive electrode active material layer (30) are directly stacked, the resistance can be improved. In addition, the wet positive electrode active material layer (20) has good adhesion to the dry positive electrode active material layer (30) and has good adhesion to the current collector (10), thereby also being able to improve the durability of the positive electrode.

[0170] In addition, the positive electrode manufactured in Comparative Example 1 has a structure in which the current collector (10) and the wet positive electrode active material layer (20) are stacked. Since the binder is contained inside the wet positive electrode active material layer (20), there is a problem of increased resistance.

[0171] In addition, the positive electrode manufactured in Comparative Example 2 has a structure in which the current collector (10) and the dry positive electrode active material layer (30) are stacked. Since a binder is applied between the current collector (10) and the dry positive electrode active material layer (30), there may be a problem that the contact resistance increases due to the binder and the high rate characteristics deteriorate.

[0172] In addition, as shown in Table 1, Figure 2a 、 2b As shown in Figures 3 and 4, it can be seen that Example 1 (a lithium-sulfur secondary battery comprising a positive electrode having a dry-type positive electrode active material layer and a wet-type positive electrode active material layer formed therein) is superior to Comparative Example 1 (a lithium-sulfur secondary battery comprising a positive electrode having a wet-type positive electrode active material layer formed therein) in terms of capacity, high-rate characteristics, overvoltage, and life characteristics.

[0173] Furthermore, it can be seen that Example 1 is superior to Comparative Example 2 (a lithium-sulfur secondary battery comprising a positive electrode having a dry-type positive electrode active material layer formed thereon) in terms of high-rate characteristics, overvoltage, and life characteristics. The reason why the capacity difference between Example 1 and Comparative Example 2 at 0.1C is not large is due to the low charge / discharge rate at which the resistance effect is not significant. That is, Figure 2b As shown, it can be seen that when the discharge rate is 0.2C, the capacity of Comparative Example 2 drops sharply due to the resistance.

[0174] [reference numerals]

[0175] 10: Current Collector

[0176] 20: Wet positive electrode active material layer

[0177] 30: Dry positive electrode active material layer

Claims

1. A positive electrode for a lithium secondary battery, comprising: positive electrode current collector; a wet positive electrode active material layer formed on one surface of the positive electrode current collector; and a dry-type positive electrode active material layer formed on the wet-type positive electrode active material layer, wherein the wet positive electrode active material layer comprises a sulfur-carbon composite, a binder and a conductive material, and The dry-type positive electrode active material layer is composed of a carbon-containing sulfur melt, which is formed by dispersing a porous carbon material in the sulfur melt.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein the wet positive electrode active material layer comprises: 40% to 80% by weight of the sulfur-carbon composite, 1% to 30% by weight of said binder, and 0.5 wt% to 30 wt% of the conductive material.

3. The positive electrode for lithium secondary battery according to claim 1, wherein the loading capacity of the wet positive electrode active material layer is 0.1 mAh / cm 2 Up to 0.5 mAh / cm 2 ,and The loading amount of the wet-type positive electrode active material layer is 2% to 20% of the loading amount of the dry-type positive electrode active material layer. 4 . The positive electrode for a lithium secondary battery according to claim 1 , wherein the porosity of the wet positive electrode active material layer is 30% to 90%.

5. The positive electrode for lithium secondary battery according to claim 1, wherein the density of the wet positive electrode active material layer is 0.2 g / cm 3 Up to 1.4g / cm 3 . 6 . The positive electrode for a lithium secondary battery according to claim 1 , wherein the porosity of the dry positive electrode active material layer is 68% or less. 7 . The positive electrode for a lithium secondary battery according to claim 1 , wherein the dry positive electrode active material layer has an internal adhesive force of 10 gf / cm or more.

8. A method for manufacturing the positive electrode for a lithium secondary battery according to claim 1, comprising the following steps: (1) coating a positive electrode slurry containing a sulfur-carbon composite, a binder, and a conductive material on one surface of a positive electrode current collector to form a wet positive electrode active material layer; and (2) A dry-type positive electrode active material layer as a self-standing film-type positive electrode material is attached to one surface of the wet-type positive electrode active material layer.

9. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 8, wherein the dry positive electrode active material layer as a self-standing film type positive electrode material is prepared by a process comprising the following steps: (a) mixing sulfur with a porous carbon material; (b) heat-treating the mixture formed in step (a); and (c) The sulfur-carbon composite formed in step (b) is placed in a container and then pressurized to form a carbon-containing sulfur melt. 10 . The method for producing a positive electrode for a lithium secondary battery according to claim 9 , wherein the heat treatment is performed at a temperature of 130° C. to 170° C. 11 . The method for producing a positive electrode for a lithium secondary battery according to claim 9 , wherein the pressurizing is performed under a pressure condition of 0.8 MPa to 15 MPa.

12. A lithium secondary battery comprising: The positive electrode for a lithium secondary battery according to claim 1; a negative electrode comprising lithium metal or a lithium alloy; a separator located between the positive electrode and the negative electrode; and An electrolyte impregnated with the positive electrode, the negative electrode, and the separator. 13 . The lithium secondary battery according to claim 12 , wherein the lithium secondary battery is a lithium-sulfur secondary battery.

14. The lithium secondary battery according to claim 12, wherein the positive electrode has a capacitance of 3.0 mAh / cm 2 Up to 5.0 mAh / cm 2 of sulfur loading.

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